1. Field of the Invention
This invention relates generally to electrical fault detectors, and is more particularly directed to a novel fault detecting circuit for a double-filament lamp which is capable of discriminating a break in one of the filaments according to a variation in the remaining filament's resistance values due to a change in temperature coefficient of resistance when one filament is broken.
2. Description of the Prior Art
Heretofore, there has been no apparatus available which can detect a filament break in a double-filament lamp by sensing the variation in resistance values due to changes in the temperature coefficient of resistance of the filaments when one of the filaments is broken.
In an example filament break or disconnection detecting circuit 1 of a prior art device shown in FIG. 1, for instance, the input portion of a first comparator 4 is connected to both ends of a detecting resistor 3 inserted and connected in series with one filament 2a of a double-filament lamp 2. Similarly, the input portion of a second comparator 6 is connected to both ends of a detecting resistor 5 inserted and connected in series with the other filament 2b of the double-filament lamp 2. A break in either filament 2a or 2b results in a voltage drop across the respective resistor 3 or 5 which is sensed by the respective comparator 4 or 6 to activate a warning lamp 7 or 8.
However, it would be inconvenient and cumbersome to insert and connect a detecting resistor such as resistor 3 or 5 in series with the filaments 2a and 2b of the aforesaid double-filament lamp 2. In addition, there is the disadvantage that an adequate intensity of illumination of the double-filament lamp 2 may not be assured due to the voltage drop caused by the detecting resistors 3 and 5.